Field of Invention
For the past one hundred years analog transmission has dominated all communication. In particular, the telephone system was originally based entirely on analog signaling. While the long-distance trunks are now largely digital in more advanced countries, the local loops are still analog and are likely to remain so for a least a decade or two, due to the enormous cost of converting them. Consequently, when a computer wishes to send digital data over a dial-up line the data must first be converted into analog form by a modem for transmission over the local loop, then converted to digital form for transmission over the long-haul trunks, then back to analog over the local loop at the receiving end and finally back to digital by another modem for storage in the destination computer.
Integrated services digital network (ISDN) can be viewed as an evolutionary progression and a conversion of analog telephone systems into an eventual all-digital network with both voice and data to be carried end-to-end in digital form. As it currently exists, the telephone company backbone and switching equipment is devoted to voice traffic over what is called a narrow band ISDN (N-ISDN) backbone.
The ISDN network architecture is based on standards set by the International Telecommunications Union (ITU) with standards in the United States largely driven by American National Standards Institute (ANSI). Two types of ISDN service are available: Basic Rate ISDN (BRI) and Primary Rate ISDN (PRI). BRI delivers two B-channels, each having a capacity of 64 Kbps and a 16 Kbps D-channel. PRI, i.e. T1, provides twenty three B-channels of 64 Kbps and a 64 Kbps D-channel. The D-channel channel is used for signaling between the central office switch and terminating equipment which could be a telephone set, personal computer, videoconferencing set or other device. The B-channels are used for any kind of service including voice data and video.
In the early days of the telephone, the connection was made by having an operator plug a jumper cable into the input and output sockets. An important property of this circuit switching is the need to set up an end-to-end path before any data can be sent. The elapsed time between the end of dialing and the start of ringing can approach ten seconds or more on a long-distance or international call. During this time interval the telephone system is hunting for a signal path. Once the setup is completed the only delay for data is the propagation time for the electromagnetic signal, about 5 msec per thousand kilometers. Once the complete end-to-end circuit is established for each pair of voice and data users the circuit is dedicated for the full duration of a call. The circuit is disconnected when either party hangs up.
To combine multiple telephone calls the telephone company uses a multiplexing technique called time division multiplexing (TDM). TDM operates on digital data. Voice digitization is accomplished by a technique known as pulse code modulation (PCM) in which an incoming analog voice conversation is encoded into a 64 Kbps digital data stream for transmission on telephone company digital transmission facilities. Once a call is digitized and routed to the distant telephone company office serving the destination or called party the 64 Kbps digital data stream is converted back to an analog voice signal and passed to the called party. Similarly the voice conversation generated by the called party flows in an analog form via the local loop to the telephone company office serving the subscriber. At that location the conversation is digitized and multiplexed using TDM equipment and placed onto a digital trunk linking that office to the office serving the call originator. At that office the call is removed from the trunk by the processing known as demultiplexing, converted back into its analog format and passed to the local subscriber.
Under the T1 format TDM occurs in frames containing 24 voice sessions. Each of the 24 groups places 8 bits into each frame for a total of 192 bits, plus an additional synchronization bit. One frame is transmitted every 125 microseconds for a total of 8000 frames per second. The frame is identified as a DS1 and each of the 24 channels within it a DS0. Thus, the T1 operating rate can be expressed as 24 channels, i.e. DS0s, times 64 kbps/channels resulting in an operating rate of 1.544 Mbps. A telephone call is established by dedicating a specific channel, i.e. a DS0. The DS0 is dedicated to a call for the full duration of the session.
One of the major limitations associated with PSTN circuit-switching is a permanent assignment of a path for the exclusive use of communication sessions during the duration of that session. This means that regardless of whether a full circuit or channel within a circuit is used to establish a path that circuit or channel cannot be used to support other activities until the session in progress is completed. The disadvantage to circuit switching particularly for a data transmission is that bandwidth must be permanently allocated for the duration of the session even though data traffic is bursty rather than continuous.
Recognizing the limitations of circuit-switching, packet switching was developed as a technique to enable the sharing of transmission facilities among many users. To overcome this limitation and to approximate over telephone networks the capabilities that existed over local area networks (LAN) packet switching over public data networks (PDN) was developed. Currently, packet switching exists as an add on to existing circuit-switched PSTN infrastructure.
Packet-based PDN utilize the existing telephone company backbone, e.g. T1 lines, for the transmission of packets of data
Packet switching also takes advantage of the burstiness of digitized speech and data communications. User information streams are divided into segments that are combined with headers to form packets. A packet switching network routes the packets to their appropriate destinations based on addresses contained in the packet headers. In this way, the network resources are statistically shared among all users, than being dedicated to users on a full-time basis. An inherent characteristic of packet switched networks is large and variable end-to-end delays that may be experienced by individual packets during transport. Sampled analog signals will only be useful for the transfer of the embedded user data with adequate additional buffering. Another problem is that packets can be delivered out of order to a destination if corrective measures are not taken. Packet network use is normally more economical than transmission over the dedicated switched circuits of traditional public-switched telephone network. Transmission facilities used to route packets from the originator to the destination examine the destination of packets, as they flow through the network and transfer the packets onto trunks based upon the packet destination and network activity.
The PSTN as it currently exists has a digital backbone that carries both packet and circuit switched traffic but the bulk of the traffic on those circuits originates over analog plain old telephone service (POTS) lines. POTS subscriber lines carry both voice and data traffic. The voice traffic enters the analog line largely unchanged, i.e. in its original analog form. There is however a basic incompatability between the digital signals transmitted by computers and those transmitted by POTS lines which were originally designed to carry analog voice signals. Although digital signals can be transmitted on an analog telephone line, their transmission distance is limited by the effects of resistance, inductance, and capacitance on the line. In addition amplifiers on the telephone lines rebuild signals literally, including any distortion experienced during transit. Thus distortion to a digital pulse is increased by the amplifier. Digital networks by contrast use regenerators that detect the incoming bit stream and create an entirely new signal that is identical with the original.
Due to the incompatabilities between the digital signals produced by computers and the analog signals POTS lines carry, a conversion device is required to enable digital signals to be carried on an analog transmission medium. The conversion device is a modem, whose name comes from a contraction of the term modulator-demodulator even though modulation and demodulation are no longer used in some types of modem. The transmitter converts the digital signals generated by computers into analog tones for transmission over telephone network analog facilities, the receiver portion of the modem receives a transmitted analog signal and reconverts it to its original digital form.
POTS analog lines are utilized to handle increasing quantities of digital data, originating from individuals and corporations. This increased demand has required a proportionate investment by the phone companies to handle the data traffic. One area in which enormous investment has been required is in linking internet service providers (ISPs) with their clients across the PSTN backbone. The required investment in this area has been increased as a result of regulations which partially exempt the ISPs from the full cost of PSTN connections. Because the ISPs do not therefore pass the full cost of PSTN connection on to their clients, i.e. they bill clients at flat rates, the clients engage in practices which waste the limited PSTN resources. One of these practices involves a client dialing in to an ISP and setting up a call session of indefinite duration. During large portions of the call session an idle condition exists, i.e. no data is being transmitted. Nevertheless the client, who is billed a flat rate, has no incentive to disconnect. Both the phone company and the ISP are disadvantaged by the practice. The ISP must lease more lines to the local office. The phone company must invest in additional resources to process the call sessions, and currently idle and active call sessions require the same amount of resources to process.
What is needed are ways to increase data handling capabilities of ISPs and the PSTN without an increase in the size of modem facilities.